1. Field of the Invention
This invention relates to a fuel supply control system for an internal combustion engine, and more particularly to a fuel supply control system using feedback control on the basis of an electrical signal obtained from a sensor placed in the exhaust pipe.
2. Description of the Prior Art
The conventionally known fuel supply control system for an internal combustion engine employs a method in which a sensor sensitive to a particular concentration of fuel gas is placed in an exhaust pipe and the air-to-fuel ratio is controlled by feeding the output signal of the sensor back to a fuel supply control apparatus. According to one of the most current methods of this kind, the partial pressure of oxygen in the exhaust gas is measured by using a solid electrolyte such as zirconium oxide and the air-to-fuel ratio is controlled to be set at a value corresponding to the chemically quantitative equilibrium point (hereafter referred to as the theoretical air-to-fuel ratio) determined from the measured partial pressure.
The output of an oxygen sensor indicates whether the air-to-fuel ratio is on the high fuel concentration side or on the low fuel concentration side. According to the output of the oxygen sensor, the control of a fuel supply is performed, correcting the rate of fuel being supplied and approximating the rate always to the theoretical air-to-fuel ratio. In practice, however, some period of time is required from the instant that fuel is supplied from the fuel supply apparatus to the combustion chamber of an engine to be burned therein till the instant that the burned fuel as exhaust gas reaches an oxygen sensor.
Since this period (hereafter referred to as dead time) is rather long, e.g. 0.5-0.1 sec, the actual air-to-fuel ratio will deviate from the theoretical air-to-fuel ratio when the oxygen sensor indicates that the corresponding air-to-fuel ratio has just coincided with the theoretical one.
As described above, according to the conventional method, since the air-to-fuel ratio is obtained from the partial pressure of oxygen in the exhaust gas, there exists a dead time due to the flow of fuel into the combustion chamber, the burning of the fuel injected, and the exhaustion of the burnt gas, as well as the response delay inherent to the feedback control in general. If the loop gain in the control system is raised, hunting of the air-to-fuel ratio as the controlled variable may result, which in turn causes a very unstable state of combustion resulting in an increase in the quantity of harmful waste gases. On the other hand, if the loop gain is greatly suppressed, the response speed of the system becomes very low so that the system will be unadaptable for the use with an internal combustion engine having large transient changes in its operation. In addition, the air-to-fuel ratio fluctuates to increase the harmful waste gases. It is therefore necessary to perform control with the loop gain maintained below the limit at which the system begins hunting, but it is still difficult to choose an adaptive gain since the delay time of the system and the dead time vary depending largely on the state of operation of the engine.